The disclosure relates generally to optical systems for telescopes, and more specifically to diffraction limited high angular resolution optical systems having large light collection diameters compared to their depth. The example application put forth in the disclosure is for astronomical telescopes.
Large astronomical telescopes have large primary light gathering mirrors in the form of a surface that is usually a section of a paraboloid, (parabola rotated about its axis). The primary may be a single mirror or a compound mirror composed of smaller sections, but the basic surface shape has two dimensional curvature. To support the primary mirror, large movable supporting structures, sometimes referred to as superstructures, are needed. To point to and track objects of interest, such as stars, the entire telescope, including the primary and secondary mirrors, must be physically moved by the supporting superstructure. Moving and accurately positioning such large and heavy mirrors within a superstructure that also rotates involves complicated actuator and control systems. The mirrors, the supporting structures, and the mechanisms are massive, and are costly to construct and maintain. For example, efforts are currently being made to construct telescopes having collection diameters as large as thirty meters, (98 feet). The width of the supporting superstructure will be forty or more meters and have a similar or greater height.
The function of a telescope is two fold. First, it is to collect a sufficient amount of light to be able to see, (or photograph), dim often distant objects. Second, it is to focus the light to from an image to be seen or photographed. The size, or diameter, of the telescope matters for both tasks, because the amount of light collected depends on the collection area, and the degree to which the light can be focused depends, among other things, on the diffraction limit set by the primary's diameter. Off axis aberrations also affect image resolution, and these are reduced by using a long focal length in comparison to the primary's diameter, (large f/#). This makes the parabolic shape closer to that of a section of a circle, but it puts the secondary and image plane far away, leading to a large telescope depth, (or height). Existing astronomical telescopes simultaneously both gather the light and focus it by using primary mirrors having compound curvature surfaces. Although the light gathering and light focusing could be separated, there is no advantage in doing this using a single primary surface. To illustrate, the light gathering could be done by using paraboloid sections for the primary and secondary mirrors aligned on their axes with a common focus. The primary gathers axial parallel light over a large collection area, and compresses it into a comparatively smaller area of axial parallel light after reflection from the secondary, conserving radiance in the process. This could then be focused by a smaller telescope. The mirrors, superstructure, and mechanisms needed, however, would be no less massive, complex, or costly than for the existing telescope.
Large astronomical telescopes, ten meters or more in diameter, have become practical due to adaptive optics. Adaptive optics removes atmospheric distortion by making fine adjustments in the positions of mirrors in the light path up to thousands of times per second. As large telescopes move to track objects, other slower rate adaptive adjustments are made to the main mirrors to keep their surfaces correctly positioned and objects focused. Because of the compound curvature of the primary mirror, its' geometric shape cannot be greatly adjusted by deforming it, without compression or tension stresses in the material, usually glass, becoming unacceptable. This is one reason for using many separate smaller sections to form a primary mirror. The depth of existing astronomical telescopes is often reduced by utilizing folded optical paths to accommodate the long focal lengths. To accommodate different observational tasks, the optical paths are often movable. The size and weight of such telescopes, however, remains substantial, and the physical depth required just to point the telescope in different directions, is still at a minimum the diameter of the primary. Additionally, movable and folded optical systems also often suffer from greater alignment difficulties.
Astronomical telescopes are now being built at a size once believed not possible. However, aside from the sheer cost considerations of building ever lager versions, the ability to handle the weight and the inertia of large and massive moving optical parts and their supporting structure present real constraints on telescope construction. The construction of even higher resolution astronomical telescopes may thus be limited using the current design approach. In addition, applications needing long focal length telescopes in situations that do not have the necessary physical depth, could benefit from a new design approach.